Toner unit for photoelectrostatic reproduction

ABSTRACT

Apparatus for applying toner to an electrostatic image carried by an insulating surface by contacting the surface with a toner mix comprising toner particles and magnetic carrying particles, which apparatus utilizes tandem magnetic brush units cooperatively arranged so that their adjacent surfaces move counter to one another, one of the magnetic brush units being arranged to form a magnetic brush from toner mix supplied thereto and to transfer toner mix in the magnetic brush to the other magnetic brush unit for formation of a second magnetic brush which may be brought into contact with the electrostatic image on the insulating surface.

United States Patent Hakanson [54] TONER UNIT FOR PHOTOELECTROSTATIC REPRODUCTION [72] inventor: Nils L. Hakanson, West Springfield, Mass.

[73] Assignee: The Plastic Coating Corporation, South Hadley, Mass.

[22] Filed: Nov. 28, 1969 [21] Appl.No.: 880,600

3,472,205 10/1969 Tsuchiya etal. 3,543,720 12/1970 Drexleret a1. ..ll8/637 Primary Examiner-Morris Kaplan Attomey-William J. Foley, Martin L. Faigus, John W. Kane, Jr. and John A. Weygandt [57] ABSTRACT Apparatus for applying toner to an electrostatic image carried by an insulating surface by contacting the surface with a toner mix comprising toner particles and magnetic carrying particles, which apparatus utilizes tandem magnetic brush units cooperatively arranged so that their adjacent surfaces move counter to one another, one of the magnetic brush units being arranged to form a magnetic brush from toner mix supplied thereto and to transfer toner mix in the magnetic brush to the other magnetic brush unit for formation of a second magnetic brush which may be brought into contact with the electrostatic image on the insulating surface.

4 Claims, 1 Drawing Figure PATENTEDAPR 11 I972 3,654,902

INVENTOR.

N l LS L. HAKANSON ATTORNEY.

TONER UNIT FOR PHOTOELECTROSTATIC REPRODUCTION BACKGROUND OF THE INVENTION 1. Field of the Invention This invention relates to an apparatus for applying toner to an electrostatic image and, more specifically, to a method and an apparatus for developing electrostatic images carried on an insulating surface by application of a toner thereto. The term electrostatic image is intended to include all electrostatic charge patterns, regardless of their method of formation.

2. Brief Description of the Prior Art In electrography, electrostatic images are normally formed on an insulating surface and are developed by the application of toner powder thereto. The toned'image may be fixed to the insulating surface itself where it is applied directly to an insulating coating on a copy sheet or it may be transferred in image configuration to another surface and thereafter fixed to that surface, leaving the insulating surface available for reuse.

Electrostatic images are normally formed on the insulating surface in the following manner. The insulating surface having photoelectrostatic properties is subjected to a corona discharge to impose an electrostatic charge on the photoelectrostatic insulating surface which, as mentioned above, is often in the form of a coating on the copy sheet. The corona discharge is placed on the sheet while the sheet is in darkness. The surface thus charged is then exposed to an optical image in the form of a light pattern to be reproduced. This exposure produces a latent electrostatic image on the insulating surface which is then developed or converted into a visible image by the application of a toner thereto.

It is a well-known commercial practice to utilize triboelectric systems for developing electrostatic images. Such systems involve the mixture of finely divided toner particles with larger magnetic carrier particles whereupon the toner particles are held to the surface of the carrier particles by electrostatic charges. Such electrostatic charges are created by triboelectrification in which friction between different materials leaves the materials with a certain charge, hopefully opposite in sign so that they attract one another. When the toner mix is brought into contact with the electrostatic image on the insulating surface, the attraction of the image for the triboelectrically charged toner particles overcomes the attraction of the carrier particles for the toner particles and the toner particles transfer to and remain on the imaged areas.

A number of different systems, including both liquid systems and dry systems, have been employed in the past for applying toner to electrostatic images. Among the dry systems are the cascade system and the magnetic brush system. Cascade systems depend on gravity to move the toner mix over the electrostatic image, and thus, are necessarily speed limiting. In addition, the cascade system requires extensive processing of the toner mix to insure uniformity of toner concentration therein at least in the portion which is to be cascaded over the imaged surface.

The magnetic brush system has been extensively researched in an attempt to discover a way to form an elongate magnetic brush having uniform properties which can be brought into contact with the imaged surface for development of the electrostatic image thereon. Such magnetic brushes are normally formed by magnetic attraction of the magnetic particles containing toner particles thereon through an intervening nonmagnetic surface so as to form the magnetic particles on the non-magnetic surface into a bristle-like formation of particles, referred to as a magnetic brush. While considerable flexibility has been achieved in terms of image density and contrast between imaged and non-imaged areas with magnetic brush systems, some problems still exist with the uniformity of image development across the copy sheet. This phenomenon appears to be governed by the uniformity of arrangement and distribution of toner particles in the magnetic brush as well as the uniformity of Shape and size of the magnetic brush. However, such problems become more prevalent as the speed of toning is increased, which is a characteristic trend at the present time.

In an attempt to improve the quality of the toned image without sacrificing speed of toning, a plurality of successive magnetic brushes have been brought into contact with the image by successively moving the copy sheet in contact with a plurality of magnetic brush units. However, such units are expensive and require a substantial amount of space, both of which factors are undesirable. In addition, the resulting improvement in quality was not entirely satisfactory.

It is the chief object and advantage of the present invention to provide an apparatus for applying toner to an electrostatic image carried by an insulating surface in a more uniform manner than has heretofore been possible through the use of a magnetic brush.

It is a further object and advantage of the present invention to provide an apparatus for forming a magnetic brush having more uniform properties along its length.

SUMMARY OF THE INVENTION In the apparatus of the invention, a first magnetic brush unit is provided which has a moving surface on which a magnetic brush of toner mix is continuously successively formed and disrupted. The moving surface of this first unit is adapted to advance in a first direction along a path such that successive transverse portions of the moving surface move through a toner mix pickup region. Means are provided for applying toner mix comprising toner particles and magnetic carrying particles to the moving surface at the toner pickup region. A second magnetic brush unit is provided which has a moving surface on which a magnetic brush of toner mix is continuously successively formed and disrupted. The moving surface of this second unit is arranged relative to the moving surface of the first unit so that successive transverse portions thereof advance through a position closely spaced from the moving surface of the first unit in a second direction counter to the first direction. By this arrangement, the second magnetic brush unit disrupts a portion of the magnetic brush formed on the moving surface of the first magnetic brush unit and a portion of the toner mix carried by the first magnetic brush unit is transferred to the second magnetic brush unit for application to the insulating surface. In the preferred embodiment, the second magnetic brush unit is disposed above the off-running side of the crest of the first magnetic brush unit in a position of interference with the magnetic brush formed on the moving surface of the first magnetic brush unit so as to create an elongate mixing zone between the rolls in which toner mix is accumulated and circulated, so that a magnetic brush having substantially uniform composition is formed on the moving surface of the second magnetic brush unit.

The invention also includes a method for applying a toner mix to an insulating surface carrying an electrostatic image. In the method, toner mix is formed into a first magnetic brush on a first surface moving along a first path. This magnetic brush is continuously disrupted and reformed as it passes through successive portions of the first path. The portion of the first magnetic brush most remote from the first surface is contacted with a second surface moving along a second path generally counter to the first path so as to accumulate and circulate at least a portion of the toner mix forming the first magnetic brush in a mixing region. At least a portion of the toner mix in the mixing region is then formed into a second magnetic brush on the second surface. The second magnetic brush is continuously disrupted and reformed as it passes through successive portions of the second path after which an insulating surface carrying an electrostatic image is moved into contacting engagement with the second magnetic brush.

BRIEF DESCRIPTION OF THE DRAWINGS The drawing is a cross-sectional view of a preferred embodiment of the apparatus of the invention.

DESCRIPTION OF THE PREFERRED EMBODIMENT According to the drawing, a container having an open top is partially filled with a toner mix 11. The toner mix 1 1 is a mixture of a powdered resinous toner and particles of a magnetic material such as, for example, iron, which is well-known to the art. The toner used for the direct production of copies is colored while that used for the production of a master which is to form a lithographic printing plate may or may not be colored, but is preferably colored to facilitate the visual inspection of the image. The toner may be one which acquires a positive charge in the toner mixture so that it will render visible the negative image areas of a latent electrostatic image and produces a positive copy. Alternatively, the toner may be one which assumes a negative charge in the toner mixture so that it is repelled by the negative image areas and renders visible the background areas of a latent electrostatic image so as to produce a negative reproduction of the original image.

Two magnetic brush units are disposed in tandem within the container, one of the units, indicated generally by reference numeral 12, being disposed generally above the other, indicated generally by reference numeral 13. The magnetic brush unit 13 has its lower periphery immersed in the toner mix 11 in the container 10. Alternatively, of course, other means might be employed for applying toner mix to the moving surface of the magnetic brush unit 13. The area where the toner mix is brought into contact with the moving surface of the magnetic brush unit 13 is the toner pickup region. This region may vary in size depending upon the means and method of toner application, but generally extends along the entire width of the magnetic brush unit 13 and for a circumferential distance sufficient to permit attachment of the toner mix to the moving surface to form a magnetic brush. The upper mag netic brush unit 12 has its periphery spaced apart from the periphery of the lower magnetic brush unit 13 but is relatively closely spaced therefrom as is described in greater detail below.

The magnetic brush units 12 and 13 are substantially identical in design, each one having a stationary cylindrical core 14 and a rotatable outer shell 15 which provides a moving surface upon which a magnetic brush of toner mix 11 is formed. The cylindrical core 14 is made of a non-magnetic material such as, for example, brass, and has a series of six longitudinal grooves 16 cut into its cylindrical surface at equally spaced intervals around approximately two thirds of its total circumference. These longitudinal grooves 16 carry permanent plastic magnet strips 17 which, from their cross-section indicated in the drawing, are, in effect, longitudinal horseshoe magnets, each one having its north pole 18 along one of its radially outer margins and its south pole 19 along the opposing radially outer margin.

The lengths of these permanent magnet strips 17 govern the width of the magnetic brush formed on the rotatable outer shell or sleeve 15 which comprises the moving surfaces of each of the magnetic brush units 12 and 13. Accordingly, these permanent magnets strips 17 should have a length at least equal to the width of the latent electrostatic image which is to be toned.

Preferably, the permanent magnet strips 17 are shorter than the sleeve 15 so that the end sections of the sleeve 15 adjacent the end sections of the core 14 do not form any magnetic brush thereon. This insures that the marginal areas of the insulating surface will not come into contact with the magnetic toning brush and hence will not pick up residual toner and become dirty in appearance even if they carry a residual electrostatic charge.

The stationary core 14 is mounted such that its rotary position may be adjusted and yet the core 14 may be firmly held in the desired position with respect to the permanent magnet strips 17 in its surface. One arrangement for accomplishing this is shown and described in U.S. Pat. No. 3,358,637, hereby incorporated by reference. Driving means (not shown) are connected to the sleeve 15 of the magnetic brush units to drive them at the desired rotation velocity, which varies according to the speed at which an imaged insulated surface is to be toned. The rotation of the cylindrical sleeve 15 within the toner mix 11 forms a magnetic brush 21 on the portion of its outer surface due to movement of the sleeve 15 past the magnetic strips 17 of the stationary core 14. Thus the magnetic strips 17 create magnetic fields outside of the sleeve 15 through which successive transverse portions of the sleeve 15 move, carrying toner mix 11. The magnetic brush 21 thus formed is continuously disrupted and reformed as the sleeve 15 of the magnetic brush unit 13 rotates, since it exists only on the segment of the surface which is momentarily adjacent the permanent magnet strips 17 of the stationary core 14. The portion of the sleeve 15 adjacent the circumferential portion of the core 14 which has no permanent magnet strips 17 cannot retain any toner mix in the form of a magnetic brush.

By disrupting and reforming the magnetic brush 21, the magnetic brush unit 13 creates intermixing and circulation of the toner mix 11 and allows excess toner mix 11 in certain portions of the brush 21 to be worked into adjacent areas to equalize the amount of toner mix 11 along the length of the brush 21. This action also tends to make the concentration of toner particles uniform along the length of the brush 21. lt should be understood that the brush 21 does not fall apart when it is disrupted and that no substantial portion of the toner mix 11 therein is lost. However, the bristlelike formation of the magnetic particles forming the magnetic brush 21 is altered as the alignment of the brush 21 changes with respect to the magnetic field due to the advance along the path of the surface of the sleeve 15 as the sleeve moves past each pole of each magnetic strip 17. The magnetic strips 17 are sufficiently closely spaced to insure that continuity of the toner mix 11 in the magnetic brush 21 will be maintained so that the brush 21 can be transferred intact from one magnetic field to a succes sive magnetic field on each of the magnetic brush units 12 and 13. This spacing will, of course, vary depending on the strength, size and shape of the permanent magnet strips 17 and the size of the resulting magnetic brush 21. From the above, it will be apparent that by altering the rotary position of the core, the characteristics of the brush 21 can be controlled at any desired location about the periphery of the sleeve 15 of the magnetic brush unit.

The upper magnetic brush unit 12 is preferably disposed above the off-running side of the crest of the lower magnetic brush unit 13 and has the surface of its sleeve 15 spaced relative to the surface of the sleeve 15 of the lower magnetic brush unit 13 such that the surface of the sleeve 15 of the upper magnetic brush unit 12 is in a position of interference with the magnetic brush 21 formed on the moving surface of the lower magnetic brush unit 13. Thus the surface of the sleeve 15 of the upper magnetic brush unit 12 is relatively free from toner mix as it moves toward the surface of the sleeve 15 of the lower magnetic brush unit 13. It should be so spaced from the moving surface of the lower magnetic brush unit 13 that it contacts the magnetic brush 21 thereon. Preferably it is sufficiently closely spaced to cause substantial disruption and intermixing of the toner mix 11 comprising the magnetic brush 21.

Since the two sleeves 15 are rotating in the same direction, as indicated above, their adjacent surfaces are moving in counter directions. This causes the accumulation of toner mix 11 in the region generally between the two moving adjacent surfaces of the sleeves 15 and preceding the point where the moving surface of the magnetic brush unit 12 contacts the magnetic brush 21 of the moving surface of the magnetic brush unit 13. Thus, this region comprises an elongate mixing zone 22 in which the particles of the toner mix are tumbled and circulated so that they become thoroughly mixed on a random basis.

This insures that the magnetic brush 23 fonned on the moving surface of the upper magnetic brush unit 12 will have a substantially uniform composition and physical shape. Thus,

the particles forming the magnetic brush 23 are picked out of the mixing and circulating mass of particles of toner mix 1 1 on such a basis that the magnetic brush formed therefrom has a substantially uniform mass, consistency and composition. The result is that the amount of toner actually applied to an electrostatic image carried on a insulating surface is substantially equal along the length of the magnetic brush 23.

To further insure the intermixing of the ingredients of the toner mix 11, a rotary stirring device 24 is preferably disposed in the bottom of the container generally beneath the lower magnetic brush unit 13. The mechanical stirrer 24 may comprise any suitable form, such as the stirrer 16 shown in the above-referenced U.S. Pat. No.3,358,637, which provides arms or looped elements arranged for movement through the toner mix 11.

In addition, oftentimes in sections along the length of the magnetic brush 23, toner particles will be removed from the toner mix 1 1 comprising the brush leaving a higher concentration of magnetic carrying particles. This will occur particularly where solid image portions are being toned in one section of the width of the apparatus over a considerable length of time. The excess toner mix 11 falling from the off-running side of the upper magnetic brush unit 12 will form piles of toner mix in container 10 which have a depleted concentration of toner particles. Toner mix used from these piles to form brushes will eventually result in the formation of weak images unless fresh toner mix is intermixed therewith.

In addition, it often becomes necessary to replenish the depleted toner particles in the toner mix. However, care must be taken to distribute new toner particles uniformly into the toner mix, or the portions of the toner mix having a greater concentration of toner particles will result in noticeable effects on the images being toned. In order to insure that the toner mix 11 has a uniform composition along the length of the container 10, a pair of counter rotating, closely spaced, and parallel screw mixers 25 and 26 are provided. These screw mixers are adapted to convey the toner mix 11 in container 10 to opposite ends of the container in a continuous circulatory manner.

In the embodiment shown in the drawing, a copy sheet 27 is moved by means (not shown) past a position where its lower surface containing an electrostatic image thereon contacts the magnetic brush 23 formed on the moving surface of the sleeve of the upper magnetic brush unit 12 whereby the image is toned. As mentioned previously, the copy sheet 27 may be any insulating surface capable of receiving an electrostatic image thereon. In the embodiment shown, the copy sheet 27 has a coating on its lower surface which has photoelectrostatic properties. For example, the coating may comprise finely divided particles of zinc oxide carried in an insulating resin. However, the copy sheet 27 might be replaced by an insulating surface having photoelectrostatic properties from which the toned image is to be subsequently transferred.

In operation, the apparatus of the invention enables the accomplishment of a new method for applying toner mix to an insulating surface carrying an electrostatic image. This new method insures that the magnetic brush formed thereby has more uniform properties in terms of configuration and toner concentration. In this method, the toner mix 11 is formed into the magnetic brush 21 on the outer surface of the sleeve 15 of the lower magnetic brush unit 13 and is carried along an arcuate path on the surface. The magnetic brush 21 is continuously disrupted and reformed as it passes along that path due to the movement of the sleeve 15 over successive permanent magnet strips 17 carried on the core 14. The magnetic brush 21 formed on the outer surface of the sleeve 15 of the lower magnetic brush unit 13 is then at least partially disrupted by contact by the surface of the sleeve 15 of the upper magnetic brush unit 12 which is moving generally counter thereto. This causes at least the portion of the first magnetic brush 21 most remote from the outer surface of the sleeve 15 of the lower unit 13 to be disrupted so that the particles of the toner mix 1 l forming it accumulate and circulate in a mixing zone 22 between the two magnetic brush units 12 and 13. After the particles have been tumbled and intermixed with one another, they are formed into a second magnetic brush 23 on the surface of the sleeve 15 of the upper magnetic brush unit 12. This magnetic brush 23 is also continuously disrupted and reformed as it is carried through an arcuate path upwardly toward the surface carrying the electrostatic image whereupon it is brought into contact with that surface so as to impart toner particles to the image in a manner rendering the image visible. Mixing of the particles in the toner mix 1 1 thus occurs in accordance with the method of the invention as the magnetic brush 21 is carried on the surface of the sleeve 15 of the first magnetic brush unit 13, and in the mixing zone 22 between the two magnetic brush units 12 and 13, and in the second magnetic brush 23 formed on the upper magnetic brush unit 12. It has been found that this increased amount of mixing of the particles in the toner mix 11 forms a magnetic brush having improved configuration and composition.

ln view of the above description of the invention and of the drawings, it can be seen that the invention provides a new and improved apparatus for applying toner to an electrostatic image carried by an insulating surface. Furthermore, it is apparent from the above description that the method and apparatus of the invention provide a magnetic brush having more uniform properties such as shape, size, consistency and toner concentration than has heretofore been achieved. Such a brush applies toner to an electrostatic image in a more uniform manner. Moreover, the use of the superior magnetic brush formed by the method and apparatus of the present invention enable toning of an electrostatic image at higher speeds without losing quality.

From the above description of the invention, it will be apparent that various modifications in the apparatus described in detail herein may be made within the scope of the invention. For example, the magnetic brush units 12 and 13 may differ in design from the form shown or from one another. Thus, the disposition and type of magnetic strips could be changed and the sleeve 15 could comprise endless belts of nonmagnetic material running over guides to provide moving surfaces upon which to form magnetic brushes. The arrangement of the units 12 and 13 might be varied, for example, side by side, as long as toner mix was transferred from one to the other via a mixing zone. Moreover, the toner mix 11 may be applied to the magnetic brush unit 13 by some means other than the container 10, such as, for example, some form of powder feed means feeding it onto the moving surface through a slot. It should be fully understood that although only two magnetic brush units have been shown in tandem, the method and apparatus of the invention comtemplate that a succession of additional units might be employed to even greater advantage in certain instances depending upon the parameters of the process. Therefore, the invention is not to be limited to the specific details of the apparatus described herein except as may be required by the following claims.

What is claimed is:

1. Apparatus for applying toner to an electrostatic image carried by an insulating surface by contacting said surface with a magnetic brush of toner mix comprising toner particles and magnetic carrying particles, said apparatus comprising:

A. a first magnetic brush unit having;

1. a moving surface,

2. means mounting said moving surface for advancing in a first direction along a path such that successive transverse portions of said moving surface move through a toner mix pickup region to a mixing zone, and 3. means for continuously successively forming and disrupting a magnetic brush of toner mix on said moving surface throughout substantially the entire path of movement of said moving surface from said toner mix pickup region to said mixing zone;

B. means for applying toner mix to said moving surface at said toner mix pickup region;

C. a second magnetic brush unit having;

1. a moving surface,

2. means mounting said moving surface of said second unit above the off-running side of a crest of the first magnetic brush unit for permitting successive transverse portions of the moving surface of said second unit to advance through a position closely spaced from the moving surface of said first unit in a second direction counter to said first direction to define said mixing zone in which toner mix is accumulated and circulated, and

3. means for transferring a portion of the toner mix carried by said first unit to said second unit at said mixing zone to form a magnetic brush on the moving surface of said second unit prior to application to said insulating surface.

2. Apparatus according to claim 8, wherein at least one of said magnetic brush units has a cylindrical moving surface.

3. Apparatus according to claim 8, wherein each of said first and second magnetic brush units has a cylindrical moving surface, and wherein said means for applying toner mix comprises a container which is adapted to hold a body of toner mix in contact with the lower part of the cylindrical moving surface of said first magnetic brush unit.

4. Apparatus according to claim 1, including mixing means disposed in said container in a position beneath the offrunning side of the crest of said second magnetic brush unit, whereby excess toner mix forming the magnetic brush which contacts said electrostatic image is mixed into said supply of toner mix in said container upon being dislodged from the moving surface of said second magnetic brush unit. 

1. Apparatus for applying toner to an electrostatic image carried by an insulating surface by contacting said surface with a magnetic brush of toner mix comprising toner particles and magnetic carrying particles, said apparatus comprising: A. a first magnetic brush unit having;
 1. a moving surface,
 2. means mounting said moving surface for advancing in a first direction along a path such that successive transverse portions of said moving surface move through a toner mix pickup region to a mixing zone, and
 3. means for continuously successively forming and disrupting a magnetic brush of toner mix on said moving surface throughout substantially the entire path of movement of said moving surface from said toner mix pickup region to said mixing zone; B. means for applying toner mix to said moving surface at said toner mix pickup region; C. a second magnetic brush unit having;
 1. a moving surface,
 2. means mounting said moving surface of said second unit above the off-running side of a crest of the first magnetic brush unit for permitting successive transverse portions of the moving surface of said second unit to advance through a position closely spaced from the moving surface of said first unit in a second direction counter to said first direction to define said mixing zone in which toner mix is accumulated and circulated, and
 3. means for transferring a portion of the toner mix carried by said first unit to said second unit at said mixing zone to form a magnetic brush on the moving surface of said second unit prior to application to said insulating surface.
 2. means mounting said moving surface for advancing in a first direction along a path such that successive transverse portions of said moving surface move through a toner mix pickup region to a mixing zone, and
 2. means mounting said moving surface of said second unit above the off-running side of a crest of the first magnetic brush unit for permitting successive transverse portions of the moving surface of said second unit to advance through a position closely spaced from the moving surface of said first unit in a second direction counter to said first direction to define said mixing zone in which toner mix is accumulated and circulated, and
 2. Apparatus according to claim 8, wherein at least one of said magnetic brush units has a cylindrical moving surface.
 3. Apparatus according to claim 8, wherein each of said first and second magnetic brush units has a cylindrical moving surface, and wherein said means for applying toner mix comprises a container which is adapted to hold a body of toner mix in contact with the lower part of the cylindrical moving surface of said first magnetic brush unit.
 3. means for transferring a portion of the toner mix carried by said first unit to said second unit at said mixing zone to form a magnetic brush on the moving surface of said second unit prior to application to said insulating surface.
 3. means for continuously successively forming and disrupting a magnetic brush of toner mix on said moving surface throughout substantially the entire path of movement of said moving surface from said toner mix pickup region to said mixing zone; B. means for applying toner mix to said moving surface at said toner mix pickup region; C. a second magnetic brush unit having;
 4. Apparatus according to claim 1, including mixing means disposed in said container in a position beneath the off-running side of the crest of said second magnetic brush unit, whereby excess toner mix forming the magnetic brush which contacts said electrostatic image is mixed into said supply of toner mix in said container upon being dislodged from the moving surface of said second magnetic brush unit. 